super cute photo & fun colors - some ways you can make spice show up better is by outlining it with a marker (always work from the back), writing in glitter glue on top, or cutting a shadow letter for behind maybe in white to match

Great color combo & cute pic! As far as the letters - maybe outline them with a white or green pen?? Or if you can still peel them up, you could try a thin layer of white paint to go behind them to tone done the bg and give them something to stand out against.